Recent advances in micro-electromechanical system (MEMS) technologies resulted in the successful commercialization of high-performance sensors and actuators in a variety of areas such as motion sensing, wireless communication, energy harvesting, and healthcare. There is a growing need for MEMS sensors with better resolution, lower noise, better bias stability, and larger dynamic range as well as a MEMS actuator with larger actuation range and better long term stability.
The performance of MEMS sensors and actuators are limited by their materials and structures. Most MEMS devices are made of silicon, metals, or polymers, which have low mechanical and optical quality factors (Q). Because of their low Q, they have large noise and small actuation range. One of the common sensors that require very high Q is a vibratory rate-integrating gyroscope (RIG). Compared to the conventional gyroscope, called the rate gyroscope (RG), the RIG offers several advantages including direct angular readout, large bandwidth, and large full-scale range. The accuracy of the RIG is roughly inversely proportional to its decay time constant (τ). The τ of silicon is limited by thermoelastic damping (TED) mechanism to less than 100 seconds, comparing to several hundred seconds for an ultra-high-Q material such as fused silica (FS). For this reason, it is desirable to fabricate the RIG using such materials. Most MEMS devices have two dimensional (2D) geometries because of limitations in existing microfabrication technologies. Sensors with these geometries tend to have worse performance under external vibrations, shocks, and temperature drifts than sensors with three dimensional (3D) geometries.
The hemispherical resonator gyroscope (HRG), developed by Delco in 1980s, is the gyroscope with the state-of-art accuracy, sufficient to guide airplanes and space satellites. The HRG is made with fused silica, which is a material with an extremely high Q due to very low TED. The HRG has a shape of a wineglass. The advantage of the wineglass geometry is that due to the symmetry of its structure, the vibrating energy does not leak through the anchor. Because of this, the sensor has a very high Q. Another advantage of the wineglass geometry is that, due its high rigidity in tilting and vertical directions, it has good vibration insensitivity. The problem of the HRG, however, is that because fused silica is very difficult to machine using the conventional micromachining technique, the wineglass resonator is made using manual grinding and polishing techniques and assembled to the electrodes. This manufacturing process is expensive, slow, and inaccurate for making micro-scale sensors.
Thus, there remains a need for micro fabrication techniques to fabricate 3D micro sensors with reflowable materials with good accuracy and extremely good surface smoothness (i.e., RMS roughness (Ra)<1 nm). This section provides background information related to the present disclosure which is not necessarily prior art.